1. Field of the Invention
The present invention relates to a thin-film transistor circuit and a liquid-crystal display using a thin-film transistor. In particular, the present invention relates to a method of manufacturing a thin-film transistor circuit provided in an SOI where an insulation layer is disposed on an insulation substrate made of glass, quartz or the like, or a mono-crystal.
2. Description of the Related Art
There has been known a technique in which a silicon film having crystallinity is formed on a glass substrate or a quartz substrate, and a thin-film transistor is manufactured with the silicon film.
At present, the integration of a peripheral drive circuit where a drive circuit consisting of TFTs for a liquid-crystal display device is integrally formed in a periphery of a pixel matrix on a substrate instead of an LSI has been developed.
The integration of the drive circuit enables the liquid-crystal display device to be downsized and the costs to be reduced.
In such a construction, the higher speed operation of the peripheral drive circuit has been required.
However, it is difficult to obtain a required high-speed operation in the existing circuit which has been formed of high-temperature polysilicon TFTs and low-temperature polysilicon TFTs.
It has been found that required high-speed drive is obtained by addition of a process for adding metal elements that promote the crystallinity of an amorphous semiconductor layer.
However, the individual thin-film transistors obtained through the above process still suffer from such a problem that their drive speed and electric characteristic are ununiform, etc.
The present invention has been aimed to provide a method of manufacturing a thin-film transistor circuit that requires the above-mentioned high-speed operation (in general, the operation speed of several tens MHz or more).
Up to now, because the metal elements that promote crystallinity are impurities for the thin-film transistors, and cannot be completely removed even though they are removed during a process after crystallization, it has been considered that the addition of the metal elements with the minimum amount as required is desirable.
Under the above circumstances, a metal element added region 105 formed for promoting the crystallinity is so shaped as to be identical with or smaller than a semiconductor island region 101 as shown in FIG. 1B.
The metal elements as added are diffused in the form of an ellipse through a heating process as shown in FIG. 1B, to promote the crystallization of a semiconductor region.
However, the semiconductor island region 101 which has been crystallized in the conventional method as shown in FIG. 1B has a semiconductor region 101 existing in a metal element diffusion region 107. For that reason, a variation in the characteristics of the respective transistors has been found although crystallization is promoted.
The present inventors have investigated a cause of the variation in the characteristics of the transistors. As a result, the present inventors have proved that a direction along which crystal of the semiconductor island region 101 grows is not identical with a direction along which carriers move in the semiconductor island region that constitutes a thin film transistor as it is apart from a metal element added region 106.
Also, in the case of fabricating a TFT using the crystalline silicon film of this type, it is preferable that a direction along which a continuity of a crystal structure extend is made nearly identical with a direction in which a source region is coupled to a drain region.
This is because it is of a structure in which the carriers are most liable to be moved during the operation of the TFT. In other words, the continuity of a lattice structure is substantially kept in the direction along which the continuity of the crystal structure extends so that scattering and trapping of the carriers as moved in the above direction is little generated in comparison with those in other directions.
As mentioned above, the characteristics of the manufactured TFT is determined in accordance with how to take the direction along which the continuity of the crystal structure extends and the direction along which the carriers move, and when both the directions are nearly identical with each other, a TFT which is high in mobility can be obtained.
Therefore, in the case where a circuit that requires a high-speed operation is manufactured using the above TFT, it is important to design the circuit in such a manner that it is arranged taking into consideration a direction of coupling the source region and the drain region of the TFT (a direction in which the carriers move during operation) with respect to a region to which the above-mentioned metal elements such as nickel are added.
In other words, crystal growth is progressed in a direction in parallel to a substrate from the region to which nickel elements are added toward the periphery of that region. This direction is a direction along which the continuity of the crystal structure extends, and the TFT is required to be arranged so that this direction is nearly identical with the direction of coupling the source region and the drain region of the TFT.
As a pattern in which the nickel added region and the TFT are arranged on a substrate as mentioned above, there is proposed an arrangement shown in FIG. 8.
In FIG. 8, TFTs 801 to 808 are disposed at the side of a nickel added region 811 or 812, and an active layer of each TFT is constituted by using a crystalline silicon film (a crystal growth region) which has grown from the nickel added region disposed at a position nearest to the TFT. Arrows indicated by reference numerals 821 to 824 denote directions of crystal growth.
However, a distance of crystal growth depends on a distance between the respective nickel added regions, and the distance of crystal growth tends to be shortened more as the distance between the respective nickel added regions is long. A difference in the distance of growth influences the characteristics of the TFTs.
A large number of circuits of the same type are built in the liquid-crystal display device. In order to drive pixels of several hundreds×several hundreds in the same manner, those large number of circuits of the same type must operate in the same manner. Therefore, the characteristics of the TFTs that constitute the respective circuits need to be unified.
In particular, there is a case where the distance of crystal growth of a silicon film used for an active layer is insufficient, or where crystallinity is insufficient, in the TFTs 801, 802, 807 and 808 formed in regions which are not interposed between the respective nickel added regions. In this case, the mobility of the TFTs 801, 802, 807 and 808 become insufficient.
In other words, comparing the TFT 801 with the TFT 1803, even if they are of the same channel conductive type and of the same dimensions, the TFT 803 reflects that crystallinity is excellent and has the characteristics suitable for high-speed operation as much, and the other TFT 801 reflects that crystallinity is not excellent and has the characteristics unsuitable for high-speed operation.
In the case of fabricating a circuit that requires high-speed operation using the above TFT, there is a case where a sufficient operation speed cannot be obtained because of a poor balance of the element characteristics.